Correspondence.
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The Editor invites correspondence on all subjects ronneetect with the use of commercial motors. Letters shozi!el be on one side of the paper .only, and type-written by Preferenc!. The right of abbreviation is reserved, and no responsibility for the views expressed is accepted.
Motor Wagon or Tractor?
The Editor, " TIIR COMMERCIAL MOTOR."
-Sir :—In one of your leaderettes of the gth instant, you state in effect that a tractor realising four tons of back-axle weight has seriously damaged a tar-macadam surface, when, under parallel conditions, a motor wagon with double the back-axle load has done no damage at all, Now, Sir, while I cannot doubt your facts, I venture to dispute an inference which is, I think, directly opposed to anything experienced in—at all events—this part of England. Assuming the tire width to be the same (say, to inches) in both cases, it is inconceivable that a vehicle realising only 4 ewt. per inch can do more damage than one giving 8 cwt. per inch, under corresponding conditions, although, considered purely as a road roller, the heavier vehicle must of necessity he the more visibly effective. Road damage may be caused either by surface shearing or scarification resulting from wheel slip, or by surface bending under a greater load per inch breadth of tire than the road strength can stand up against, and, as your fuggestion is that the damage has been of the former character, I think the reason must be sought for either in tire dissimilarity or in the different speeds of the contrasted vehicles, for, as regards the latter' most metallic-tired self-propelled vehicles slip substantially less on the slow, than when running -on the fast speed. I should therefore like to ask the following questions :—
f. Were the tires in both cases smooth, and of similar section ; or, if interrupted, was the creeper-plate spacing identical?
2. Was the tire material identical in hardness? A soft iron or dead-mild steel tire obtains, of course, a very much better surface grip than one of chilled iron or hard steel.
3. Were the atmospheric conditions in both cases the same? A few degrees in temperature make a material difference in the plasticity and abrasive resistance of tar macadam, and the same applies to the amount and condition, • at the time, of the grit with which tarmacadam is usually top-dressed.
I am not prepared to say that, in driving-wheel effect on road surfaces, a five-ton back load is exactly the same as a five-ton trailing one; but, at the same speed, the percentage of driving-wheel slip must surely be greater in the 3-foot to 31,-foot, motor-wheel. diameter, than in the. 41-foot to 5-foot of the tractor. As a matter of fact, it is extremely difficult to speak categorically of self-propelled traffic comparative effects on macadam of any kind, owing to its extreme fluctuation of composition, surface, foundation, and drainage, etc., to say nothing of the variable weights of the steam rollers used in its formation. It is in the case of paving-sett traffic that one most clearly observes the differences in tire character, and wheel diameter, and the gross unfairness of the present position lies in the fact that, while the law is restrictive in a very absurd way as to the character of driving-wheel tires, there is hardly any restriction whatever on that of the road surfaces upon which they have to operate. For example, a tractor may only carry 3 to 4 tons on a pair of to-inch tires, a motor wagon may convey 8 tons on the same, whilst a traction engine to carry 8 tons must have at least 4-inch driving-wheel tires—and all have to be effective on at least 20 different kinds of street and highway surface.—I am, Sir, yours faithfully,
jours MORRISON.
[4 copy of the above letter was sent to Mr. Jas. H. Mann, of Leeds, who had informed us of the observations u¢on which we commented. and the following letter has been received by us in response.—En.]
The Editor, " TIIE COMMERCIAL MOTOR."
Sir :—With regard to relative damage done to a stretch of tarred macadam by one of our steam carts carrying a load of 5 tons and a tractor pulling the same load on a trailer behind, as I personally witnessed the passage of the two machines over the macadam, there is no possible question as to the main facts.
On the day in question, both the cart and tractor left our works together, and kept together until within a few miles of the hill in question. The cart was then allowed to go forward, as it was found capable of making a better average speed than the tractor. It, therefore, reached the macadam possibly half an hour, in advance of the tractor, and about tea-time. On arriving at the tarred macadam with the tractor, I had the greatest difficulty in determining actually where the cart had gone over it ; the marks left were so slight as to be hardly distinguishable. Both the cart and tractor were fitted with steel tire plates of exactly the same quality of steel, fastened to the rims in exactly the same manner, the tire plates in each case were of the same age, and in each case had space between adjacent plates in accordance with the Heavy Motor Car Order. The atmospheric conditions were quite unchanged, the tractor following the cart—as before stated---at about half an hour's interval, and there was no change in the weather in the .meantime. The tar macadam stretch was left entirely unguarded, being, therefore, presumably considered fit for traffic, and it had obviously been thoroughly rolled.
We quite expect you will find many people refuse to believe the relative destructive action of the tractor when it is pulling a load behind, as compared with the heavy axle-load of a machine carrying an equal weight, and road surveyors, like the general public, hold a view that the tractor does the least damage. We consider the present opinion is largely a matter of presumption, and that one undoubted demonstration such as this should carry a great deal of weight as compared with mere opinions. Furthermore, we have since been carefully watching the driving wheels of a traction engine on a macadamised road of slight gradient, and, in spite of the shoes being wide apart and not close as
on a tractor, the .wheels could be observed, even when the traction engine was moving steadily forward, to slip back most distinctly.—Yours truly,
MANN'S PATENT STEAM CART AND WAGON CO., LTD.
J. C. CORNOCK, Manager and Secretary.
Ixeds.
Reports on Sentinel Wagons.
The Editor, "THE COMMERCIAL MOTOR."
Sir :—You will remember that, some time back [Issue of the 21st of May last.—ED.], I enquired through your journal for reports on Sentinel steam wagons. I received such good replies from users on the merits of these wagons that I placed an order with Messrs. Alley and Maclellan, of the Sentinel Works, Glasgow, for a wagon—a six-Loaner : it was delivered to me within to days of my ordering.
I have driven all sorts of wagons, etc., on the road for nearly 20 years, but I confess I have never handled a better machine in -my life, or one that is more simple, and for smooth running I don't believe there is anything to beat it. The wagon is built for work ; you are not paying for a lot of fancy parts, but something well made and well put together. There is no noise and rattle to annoy anyone, as there is with a lot of them, and, as for hill climbing, she will go up anything in the West Country. I am highly pleased with her, and can well recommend a Sentinel to any party thinking of going in for a steam wagon : I am sure their doubts would be put to rest—Yours truly,
"MORE THAN PT.F.ASF.D."